Mobile communications systems are widely used today, including mobile phone systems and wireless local area networks (LAN). Many of those mobile communications systems use cellular technology, which partitions the service coverage area into a plurality of cells each containing a base station so that those base stations cooperate with each other to control communication. A mobile station in a cell is wirelessly connected to the base station of that cell. The mobile station can communicate with other mobile stations or computers via the base station.
A mobile station moving in a cellular system may encounter a situation where its connection has to be transferred from the current base station to another base station (called “handover”). Generally, the decision on whether to perform handover is made on the basis of communication quality of downlink paths. That is, the mobile station measures downlink communication quality based on a signal received from each base station, and initiates a handover operation if there is found a base station which provides better communication quality than the currently connected base station.
Conventionally the primary objective of mobile communications systems is to provide communication services over as wide an area as possible. Macro cells using high-power base stations have thus been deployed in preference to other types of cells. In recent years, however, mobile communications systems are more and more required to offer stable communication services with higher bitrates. For this reason, it is the current trend to deploy additional micro cells using low-power base stations, with the coverage overlapping with the existing macro cells.
The resulting mobile communications system thus includes base stations with different transmission powers. This system requires more complex communication control to make efficient use of radio resources. For example, the following techniques are known. One known technique selects a target station for handover in consideration of not only the magnitude of receive power from base stations, but also the power level of interference from the neighboring base stations, in order to prevent network connections from concentrating in a high-power base station (see, e.g., Japanese Laid-open Patent Publication No. 2000-270356). Another known technique suppresses the transmit power of a mobile station so as to control radio interference which could occur with the base stations surrounding the mobile station (see, e.g., Japanese Laid-open Patent Publication No. 2000-333265).
Yet another known technique selects a base station for uplink and a base station for downlink independently of each other, in the case where the communications system supports soft handover (see, e.g., Japanese Laid-open Patent Publication No. 2000-269881). Still another known technique uses a micro-cell base station for downlink and a macro-cell base station for uplink in the case where a plurality of micro cells are deployed in a macro cell (see, e.g., Japanese Laid-open Patent Publication No. 07-75154).
It is noted that none of the techniques described in the above-noted Japanese Laid-open Patent Publications Nos. 2000-270356, 2000-333265, 2000-269881, and 07-75154 provides sufficient flexibility in how to select a target base station for handover. In the actual communication context, the appropriateness of a base station selection depends also on what kind of data services (e.g., voice call, electronic mail transmission, file downloading) the mobile station uses when handover takes place. In respect of this issue, the techniques described in above-noted Japanese Laid-open Patent Publications Nos. 2000-270356, 2000-333265, 2000-269881, and 07-75154 may not always provide an optimal selection of base station.
In addition, the above-noted techniques described in Japanese Laid-open Patent Publications Nos. 2000-269881 and 07-75154 are limited in their range of applicable communications systems. For example, the technique described in Japanese Laid-open Patent Publication No. 2000-269881 requires the subject communications system to support soft handover. Further, the technique described in Japanese Laid-open Patent Publication No. 07-75154 assumes a fixed relationship between macro cells and micro cells.